Application of heat to oil shale formations is described in U. S. Patent Nos. 2,923,535 to Ljungstrom and 4,886,118 to Van Meurs et al. These prior art references disclose that electrical heaters transmit heat into an oil shale formation to pyrolyze kerogen within the oil shale formation. The heat may also fracture the formation to increase permeability of the formation. The increased permeability may allow formation fluid to travel to a production well where the fluid is removed from the oil shale formation. In some processes disclosed by Ljungstrom, for example, an oxygen containing gaseous medium is introduced to a permeable stratum, preferably while still hot from a preheating step, to initiate combustion.

U. S. Patent No. 2,548,360 describes an electrical heating element placed within a viscous oil within a wellbore. The heater element heats and thins the oil to allow the oil to be pumped from the wellbore. U. S. Patent No. 4,716,960 describes electrically heating tubing of a petroleum well by passing a relatively low voltage current through the tubing to prevent formation of solids. U. S. Patent No. 5,065,818 to Van Egmond describes an electrical heating element that is cemented into a well borehole without a casing surrounding the heating element.

U. S. Patent No. 6,023,554 to Vinegar et al. describes an electrical heating element that is positioned within a casing. The heating element generates radiant energy that heats the casing. A granular solid fill material may be placed between the casing and the formation. The casing may conductively heat the fill material, which in turn conductively heats the formation.

U. S. Patent No. 4,570,715 to Van Meurs et al., which is incorporated by reference as if fully set forth herein, describes an electrical heating element. The heating element has an electrically conductive core, a surrounding layer of insulating material, and a surrounding metallic sheath. The conductive core may have a relatively low resistance at high temperatures. The insulating material may have electrical resistance, compressive strength and heat conductivity properties that are relatively high at high temperatures. The insulating layer may inhibit arcing from the core to the metallic sheath. The metallic sheath may have tensile strength and creep resistance properties that are relatively high at high temperatures.

U. S. Patent No. 5,060,287 to Van Egmond, which is incorporated by reference as if fully set forth herein, describes an electrical heating element having a copper- nickel alloy core.

Combustion of a fuel may be used to heat a formation.

Combusting a fuel to heat a formation may be more economical than using electricity to heat a formation.

Several different types of heaters may use fuel combustion as a heat source that heats a formation. The combustion may take place in the formation, in a well and/or near the surface.

US patent Nos. 4,662,443; 4,662,439 and 4,648,450 disclose fireflooding methods to combust hydrocarbons within an underground formation, wherein an oxidizer,

such as air, is pumped into the formation. The oxidizer may be ignited to advance a fire front towards a production well. Oxidizer pumped into the formation may flow through the formation along fracture lines in the formation. Ignition of the oxidizer may not result in the fire front flowing uniformly through the formation.

It is also known to use a flameless combustor to combust a fuel that is injected into a heater well. U. S.

Patent Nos. 5,255,742 to Mikus, 5,404,952 to Vinegar et al., 5,862,858 to Wellington et al., and 5,899,269 to Wellington et al., which are incorporated by reference as if fully set forth herein, describe flameless combustors.

Flameless combustion may be accomplished by preheating a fuel and combustion air to a temperature above an auto- ignition temperature of the mixture. The fuel and combustion air may be mixed in a heating zone to combust.

In the heating zone of the flameless combustor, a catalytic surface may be provided to lower the auto- ignition temperature of the fuel and air mixture. In these known flameless combustors fuel and oxidant are injected into the heater well through separate supply conduits or as a mixture through a single supply conduit, whereas the exhaust gases are vented to surface via an exhaust conduit which may co-axially surround the fuel and/or oxidant supply conduit (s).

It is also known to supply heat to a formation from a surface heater. The surface heater may produce combustion gases that are circulated through wellbores to heat the formation. Alternately, a surface burner may be used to heat a heat transfer fluid that is passed through a wellbore to heat the formation. Examples of fired heaters, or surface burners that may be used to heat a subterranean formation, are illustrated in U. S. Patent Nos. 6,056,057 to Vinegar et al. and 6,079,499 to Mikus

et al., which are both incorporated by reference as if fully set forth herein.

A disadvantage of the known surface and downhole heaters wherein fuel, oxidant and/or exhaust gases are circulated through a heater well is that the casing and other conduits in the heater well need to be made of a high temperature resistant steel grade and in particular the casing is exposed to high compressive forces as a result of the thermal expansion of the surrounding formation. The casing in the heater well has therefore to be made of an expensive high-temperature and corrosion resistant stainless steel grade. Also the supply of fuel or, if an electrical heater is installed, supply of electrical power generally requires a complex infrastructure and is therefore expensive.

A disadvantage of the known fireflooding methods is that fractures are created in irregular patterns in the hydrocarbon containing formation and that only hydro- carbons near the fractures are oxidised, so that only the formation is heated in a rather irregular and uncontrollable manner.

It is an object of the present invention to alleviate the disadvantages of the known fireflooding, injected fuel combustion and electrical heating methods and to provide an inexpensive downhole heating method and system which transmit a controlled amount of heat in a uniform manner into the formation.

Summary of the Invention In accordance with the present invention a system for transmitting heat into a hydrocarbon containing formation surrounding a heat injection well comprises: an oxidizing fluid source; an oxidant supply conduit disposed in the wellbore of the heat injection well, wherein the conduit is configured to provide an oxidizing fluid from the

oxidizing fluid source to a reaction zone in the formation during use, and wherein the oxidizing fluid is selected to oxidize at least a portion of the hydro- carbons in the formation in the vicinity of the wellbore zone during use such that heat is generated at the reaction zone; and a combustion gases exhaust conduit disposed in the wellbore of the heat injection well for transmitting combustion gases through the wellbore of the heat injection well away from the reaction zone.

Preferably, the system is configured to allow heat to transfer substantially by conduction from the reaction zone to a selected section of the formation during use.

It is also preferred that the oxidant supply and combustion gases exhaust conduits are equipped with pressure regulation devices which control the pressure in the reaction zone such that at least a substantial part of the combustion gases generated at the reaction zone are vented to the earth surface through the combustion gases exhaust conduit.

In some cases the pressure generating device can be used to allow part of the combustion gases to exhaust to the earth surface and part to penetrate the process zone.

This may allow for a higher pressure in the wellbore than in the region away from the wellbore. This pressure differential may cause the oxidizing fluid to reach the reaction zone more rapidly and/or in larger quantities thereby allowing increased heat generation from the reaction zone.

Suitably, the oxidant injection conduit and the combustion gases exhaust conduit extend co-axially to each other from a wellhead of the heater well into the hydrocarbon bearing formation, and the oxidant injection conduit protrudes from the lower end of the oxidant injection conduit through at least a substantial part of

the hydrocarbon bearing formation and the protruding lower part of the oxidant injection conduit is equipped with an array of oxidant injection ports via which in us oxidant is injected at a subsonic or supersonic velocity into an annular space between the oxidant injection conduit and the reaction zone.

The oxidant injection conduit may be an air injection conduit and be provided with an air injection pump and the air injection conduit and combustion gases exhaust conduits may be each provided with a pressure control valve for controlling the pressure in the annular space between the perforated lower part of the oxidant injection conduit and the reaction zone such that said pressure is substantially equal to a pore pressure in at least part the surrounding hydrocarbon containing formation and transfer of combustion gases into the formation is inhibited. But in some cases partial penetration of the combustion gases into the formation may be allowed in order to accelerate the transfer of oxidising fluid to the reaction zone and increase the heat generation there.

To start up or support the in-situ combustion process the heater well may be equipped with an electric heater for transmitting heat into the reaction zone or with a fuel injection conduit for injecting additional fuel into the reaction zone.

Detailed description of the invention The invention will be described in more detail and by way of example with reference to the accompanying drawings, in which: FIG. 1 depicts an embodiment of a natural distributed combustor heat source; FIG. 2 depicts a portion of an overburden of a formation with a heat source;

FIGS. 3 and 4 depict embodiments of a natural distributed combustor heater; and FIGS. 5 and 6 depict embodiments of a system for heating a formation.

In accordance with the invention a hydrocarbon containing formation is heated by in-situ oxidation of hydrocarbons in the formation surrounding a heat injection well, which system is also referred to as a natural distributed combustor (NDC) heating system. The generated heat may be allowed to transfer by convection to a section of the formation surrounding the heater well to heat it, whereas transfer of combustion gases from the reaction zone into the formation is inhibited or partially inhibited.

A temperature sufficient to support oxidation may be, for example, at least about 200 °C or 250 °C. The temperature sufficient to support oxidation will tend to vary, however, depending on, for example, a composition of the hydrocarbons in the hydrocarbon containing formation. Water may be removed from the formation prior to heating. For example, the water may be pumped from the formation by dewatering wells. The heated portion of the formation may be near or substantially adjacent to an opening in the hydrocarbon containing formation. The opening in the formation may be a heater well formed in the formation. The heater well may be formed as in any of the embodiments described herein. The heated portion of the hydrocarbon containing formation may extend radially from the opening to a width of about 0.3 m to about 1.2 m. The width, however, may also be less than about 0.9 m. A width of the heated portion may vary. In certain embodiments the variance will depend on, for example, a width necessary to generate sufficient heat during oxidation of carbon to maintain the oxidation reaction without providing heat from an additional heat source.

After the portion of the formation reaches a temperature sufficient to support oxidation, an oxidizing fluid may be provided into the opening to oxidize at least a portion of the hydrocarbons at a reaction zone, or a heat source zone, within the formation. Oxidation of the hydrocarbons will generate heat at the reaction zone.

The generated heat will in most embodiments transfer from the reaction zone to a pyrolysis zone in the formation.

In certain embodiments the generated heat will transfer at a rate between about 650 watts and 1650 watts per meter as measured along a depth of the reaction zone.

Upon oxidation of at least some of the hydrocarbons in the formation, energy supplied to the heater for initially heating may be reduced or may be turned off. As such, energy input costs may be significantly reduced, thereby providing a significantly more efficient system for heating the formation.

In an embodiment, a conduit may be disposed in the opening to provide the oxidizing fluid into the opening.

The conduit may have flow orifices, or other flow control mechanisms (i. e., slits, venturi meters, valves, etc.) to allow the oxidizing fluid to enter the opening. The term "orifices"includes openings having a wide variety of cross-sectional shapes including, but not limited to, circles, ovals, squares, rectangles, triangles, slits, or other regular or irregular shapes. The flow orifices may be critical flow orifices through which fluids flow at a high, e. g. supersonic, speed to provide a substantially constant flow of oxidizing fluid into the opening, regardless of the pressure in the opening.

In some embodiments, the number of flow orifices, which may be formed in or coupled to the conduit, may be limited by the diameter of the orifices and a desired spacing between orifices for a length of the conduit. For example, as the diameter of the orifices decreases, the

number of flow orifices may increase, and vice versa. In addition, as the desired spacing increases, the number of flow orifices may decrease, and vice versa. The diameter of the orifices may be determined by, for example, a pressure in the conduit and/or a desired flow rate through the orifices. For example, for a flow rate of about 1.7 standard cubic meters per minute and a pressure of about 7 bar absolute, an orifice diameter may be about 1.3 mm with a spacing between orifices of about 2 m.

Smaller diameter orifices will tend to plug more easily than larger diameter orifices due to, for example, contamination of fluid in the opening or solid deposition within or proximate to the orifices. In some embodiments, the number and diameter of the orifices can be chosen such that a more even or nearly uniform heating profile will be obtained along a depth of the formation within the opening. For example, a depth of a heated formation that is intended to have an approximately uniform heating profile may be greater than about 300 m, or even greater than about 600 m. Such a depth may vary, however, depending on, for example, a type of formation to be heated and/or a desired production rate.

In some embodiments, flow orifices may be disposed in a helical pattern around the conduit within the opening.

The flow orifices may be spaced by about 0.3 m to about 3 m between orifices in the helical pattern. In some embodiments, the spacing may be about 1 m to about 2 m or, for example, about 1.5 m.

The flow of the oxidizing fluid into the opening may be controlled such that a rate of oxidation at the reaction zone is controlled. The oxidizing fluid may also cool the conduit such that the conduit is not substantially heated by oxidation.

FIG. 1 illustrates an embodiment of a natural distributed combustor configured to heat a hydrocarbon

containing formation. Conduit 512 may be placed into opening 514 in formation 516. Conduit 512 may have inner conduit 513. Oxidizing fluid source 508 may provide oxidizing fluid 517 into inner conduit 513. Inner conduit 513 may have critical flow orifices 515 along its length. Critical flow orifices 515 may be disposed in a helical pattern (or any other pattern) along a length of inner conduit 513 in opening 514. For example, critical flow orifices 515 may be arranged in a helical pattern with a distance of about 1 m to about 2.5 m between adjacent orifices. Critical flow orifices 515 may be further configured as described herein. Inner conduit 513 may be sealed at the bottom. Oxidizing fluid 517 may be provided into opening 514 through critical flow orifices 515 of inner conduit 513.

Critical flow orifices 515 may be designed such that substantially the same flow rate of oxidizing fluid 517 may be provided through each critical flow orifice.

Critical flow orifices 515 may also provide substantially uniform flow of oxidizing fluid 517 along a length of conduit 512. Such flow may provide substantially uniform heating of formation 516 along the length of conduit 512.

Packing material 542 may enclose conduit 512 in overburden 540 of the formation. Packing material 542 may substantially inhibit flow of fluids from opening 514 to surface 550. Packing material 542 may include any material configurable to inhibit flow of fluids to surface 550 such as cement, sand, and/or gravel.

Oxidation products 519 typically enter conduit 512 from opening 514. Oxidation products 519 may include carbon dioxide, oxides of nitrogen, oxides of sulphur, carbon monoxide, and/or other products resulting from a reaction of oxygen with hydrocarbons and/or carbon.

Oxidation products 519 may be removed through conduit 512 to surface 550. Oxidation product 519 may flow along a

face of reaction zone 524 in opening 514 until proximate an upper end of opening 514 where oxidation product 519 may flow into conduit 512. Oxidation products 519 may also be removed through one or more conduits disposed in opening 514 and/or in formation 516. For example, oxidation products 519 may be removed through a second conduit disposed in opening 514. Removing oxidation products 519 through a conduit may substantially inhibit oxidation products 519 from flowing to a production well disposed in formation 516. Critical flow orifices 515 may also be configured to substantially inhibit oxidation products 519 from entering inner conduit 513.

A flow rate of oxidation product 519 may be balanced with a flow rate of oxidizing fluid 517 such that a substantially constant pressure is maintained within opening 514. For a 100 m length of heated section, a flow rate of oxidizing fluid may be between about 0.5 standard cubic meters per minute to about 5 standard cubic meters per minute, or about 1.0 standard cubic meters per minute to about 4.0 standard cubic meters per minute, or, for example, about 1.7 standard cubic meters per minute. A pressure in the opening may be, for example, about 8 bar absolute. Oxidizing fluid 517 may oxidize at least a portion of the hydrocarbons in heated portion 518 of hydrocarbon containing formation 516 at reaction zone 524. Heated portion 518 may have been initially heated to a temperature sufficient to support oxidation by an electric heater, as shown in FIG. 5, or by any other suitable system or method described herein. In some embodiments, an electric heater may be placed inside or strapped to the outside of conduit 513.

In certain embodiments it is beneficial to control the pressure within the opening 514 such that oxidation product and/or oxidation fluids are inhibited from flowing into the pyrolysis zone of the formation. In some

instances pressure within opening 514 will be balanced with pressure within the formation to do so. In other embodiments some of the combustion products may be allowed to flow into the formation in order that the pressure differential so created will accelerate the flow of oxidizing fluid towards the reaction zone an thus increase the heat generation rate.

Although the heat from the oxidation is transferred to the formation, oxidation product 519 (and excess oxidation fluid such as air) may be substantially inhibited from flowing through the formation and/or to a production well within formation 516. Instead oxidation product 519 (and excess oxidation fluid) is removed (e. g., through a conduit such as conduit 512) as is described herein. In this manner, heat is transferred to the formation from the oxidation but exposure of the pyrolysis zone with oxidation product 519 and/or oxidation fluid may be substantially inhibited and/or partially or totally prevented.

In certain embodiments, some pyrolysis product near the reaction zone 524 may also be oxidized in reaction zone 524 in addition to the carbon. Oxidation of the pyrolysis product in reaction zone 524 may provide additional heating of formation 516. When such oxidation of pyrolysis product occurs, it is desirable that oxidation product from such oxidation be removed (e. g., through a conduit such as conduit 512) near the reaction zone as is described herein, thereby inhibiting contamination of other pyrolysis product in the formation with oxidation product.

Conduit 512 may be configured to remove oxidation product 519 from opening 514 in formation 516. As such, oxidizing fluid 517 in inner conduit 513 may be heated by heat exchange in overburden section 540 from oxidation product 519 in conduit 512. Oxidation product 519 may be

cooled by transferring heat to oxidizing fluid 517. In this manner, oxidation of hydrocarbons within formation 516 may be more thermally efficient.

Oxidizing fluid 517 may transport through reaction zone 524, or heat source zone, by gas phase diffusion and/or convection. Diffusion of oxidizing fluid 517 through reaction zone 524 may be more efficient at the relatively high temperatures of oxidation. Diffusion of oxidizing fluid 517 may inhibit development of localized overheating and fingering in the formation. Diffusion of oxidizing fluid 517 through formation 516 is generally a mass transfer process. In the absence of an external force, a rate of diffusion for oxidizing fluid 517 may depend upon concentration, pressure, and/or temperature of oxidizing fluid 517 within formation 516. The rate of diffusion may also depend upon the diffusion coefficient of oxidizing fluid 517 through formation 516. The diffusion coefficient may be determined by measurement or calculation based on the kinetic theory of gases. In general, random motion of oxidizing fluid 517 may transfer oxidizing fluid 517 through formation 516 from a region of high concentration to a region of low concentration. Convection of oxidizing fluid from the injection orifices to the reaction zone will be governed by the pressure differential between the wellbore and the reaction zone based on the laws of fluid flow through porous media.

With time, reaction zone 524 may slowly extend radially to greater diameters from opening 514 as hydrocarbons are oxidized. Reaction zone 524 may, in many embodiments, maintain a relatively constant width. For example, reaction zone 524 may extend radially at a rate of less than about 0.91 m per year for a hydrocarbon containing formation. For example, for a coal containing formation, reaction zone 524 may extend radially at a

rate between about 0.5 m per year to about 1 m per year.

For an oil shale containing formation, reaction zone 524 may extend radially about 2 m in the first year and at a lower rate in subsequent years due to an increase in volume of reaction zone 524 as reaction zone 524 extends radially. Such a lower rate may be about 1 m per year to about 1.5 m per year. Reaction zone 524 may extend at slower rates for hydrocarbon rich formations (e. g., coal) and at faster rates for formations with more inorganic material in it (e. g., oil shale) since more hydrocarbons per volume are available for combustion in the hydrocarbon rich formations.

A flow rate of oxidizing fluid 517 into opening 514 may be increased as a diameter of reaction zone 524 increases to maintain the rate of oxidation per unit volume at a substantially steady state. Thus, a temperature within reaction zone 524 may be maintained substantially constant in some embodiments. The temperature within reaction zone 524 may be between about 650 °C to about 900 °C or, for example, about 760 °C.

The temperature within reaction zone 524 may vary depending on, for example, a desired heating rate of selected section 526. The temperature within reaction zone 524 may be increased or decreased by increasing or decreasing, respectively, a flow rate of oxidizing fluid 517 into opening 514. A temperature of conduit 512, inner conduit 513, and/or any metallurgical materials within opening 514 usually may not, however, exceed the temperature at which the metallurgical materials will begin to deform or corrode rapidly.

An increase in the diameter of reaction zone 524 may provide relatively rapid heating of hydrocarbon containing formation 516. As the diameter of reaction zone 524 increases, an amount of heat generated per time in reaction zone 524 may also increase. Increasing an

amount of heat generated per time in a reaction zone will in many instances provide for an increased heating rate of formation 516 over a period of time, even without increasing the temperature in the reaction zone or the temperature at conduit 513. Thus increased heating may be achieved over time without installing additional heat sources, and without increasing temperatures. In some embodiments the heating rates may be increased while allowing the temperatures to decrease (allowing temperatures to decrease may often lengthen the life of the equipment used).

By utilizing the carbon in the formation as a fuel, the natural distributed combustor may save significantly on energy costs. Thus, an economical process may be provided for heating formations that may otherwise be economically unsuitable for heating by other methods.

Also, fewer heaters may be placed over an extended area of formation 516. This may provide for a reduced equipment cost associated with heating formation 516.

The heat generated at reaction zone 524 may transfer by thermal conduction to selected section 526 of formation 516. In addition, generated heat may transfer from a reaction zone to the selected section to a lesser extent transfer by convection. Selected section 526, sometimes referred to herein as the"pyrolysis zone,"may be substantially adjacent to reaction zone 524. Since oxidation product (and excess oxidation fluid such as air) is typically removed from the reaction zone, the pyrolysis zone can receive heat from the reaction zone without being exposed to oxidation product, or oxidants, that are in the reaction zone. Oxidation product and/or oxidation fluids may cause the formation of undesirable formation products if they are present in the pyrolysis zone. For example, in certain embodiments it is desirable to conduct pyrolysis in a reducing environment. Thus it

is often useful to allow heat to transfer from the reaction zone to the pyrolysis zone while inhibiting or preventing oxidation product and/or oxidation fluid from reaching the pyrolysis zone.

Pyrolysis of hydrocarbons, or other heat-controlled processes, may take place in heated selected section 526.

Selected section 526 may be at a temperature between about 270 °C to about 400 °C for pyrolysis. The temperature of selected section 526 may be increased by heat transfer from reaction zone 524. A rate of temperature increase may be selected as in any of the embodiments described herein. A temperature in formation 516, selected section 526, and/or reaction zone 524 may be controlled such that production of oxides of nitrogen may be substantially inhibited. Oxides of nitrogen are often produced at temperatures above about 1200 °C.

A temperature within opening 514 may be monitored with a thermocouple disposed in opening 514. The temperature within opening 514 may be monitored such that a temperature may be maintained within a selected range.

The selected range may vary, depending on, for example, a desired heating rate of formation 516. A temperature may be maintained within a selected range by increasing or decreasing a flow rate of oxidizing fluid 517. For example, if a temperature within opening 514 falls below a selected range of temperatures, the flow rate of oxidizing fluid 517 may be increased to increase the combustion and thereby increase the temperature within opening 514. Alternatively, a thermocouple may be disposed on conduit 512 and/or disposed on a face of reaction zone 524, and a temperature may be monitored accordingly.

In certain embodiments one or more natural distributed combustors may be placed along strike and/or

horizontally. Doing so tends to reduce pressure differentials along the heated length of the well, thereby tending to promote more uniform heating and improved control.

In some embodiments, a presence of air or molecular oxygen, 02, in oxidation product 519 may be monitored.

Alternatively, an amount of nitrogen, carbon monoxide, carbon dioxide, oxides of nitrogen, oxides of sulphur, etc. may be monitored in oxidation product 519.

Monitoring the composition and/or quantity of oxidation product 519 may be useful for heat balances, for process diagnostics, process control, etc.

FIG. 2 illustrates an embodiment of a section of overburden with a natural distributed combustor as described in FIG. 1. Overburden casing 541 may be disposed in overburden 540 of formation 516. Overburden casing 541 may be substantially surrounded by materials (e. g., an insulating material such as cement) that may substantially inhibit heating of overburden 540.

Overburden casing 541 may be made of a metal material such as, but not limited to, carbon steel.

Overburden casing may be placed in reinforcing material 544 in overburden 540. Reinforcing material 544 may be, for example, cement, sand, concrete, etc. Packing material 542 may be disposed between overburden casing 541 and opening 514 in the formation. Packing material 542 may be any substantially non-porous material (e. g., cement, concrete, grout, etc.). Packing material 542 may inhibit flow of fluid outside of conduit 512 and between opening 514 and surface 550.

Inner conduit 513 may provide a fluid into opening 514 in formation 516. Conduit 512 may remove a combustion product (or excess oxidation fluid) from opening 514 in formation 516. Diameter of conduit 512 may be determined

by an amount of the combustion product produced by oxidation in the natural distributed combustor. For example, a larger diameter may be required for a greater amount of exhaust product produced by the natural distributed combustor heater.

In an alternative embodiment, at least a portion of the formation may be heated to a temperature such that at least a portion of the hydrocarbon containing formation may be converted to coke and/or char. Coke and/or char may be formed at temperatures above about 400 °C and at a high heating rate (e. g., above about 10 °C/day). In the presence of an oxidizing fluid, the coke or char will oxidize. Heat may be generated from the oxidation of coke or char as in any of the embodiments described herein.

FIG. 3 illustrates an embodiment of a natural distributed combustor heater. Insulated conductor 562 may be coupled to conduit 532 and placed in opening 514 in formation 516. Insulated conductor 562 may be disposed internal to conduit 532 (thereby allowing retrieval of the insulated conductor 562), or, alternately, coupled to an external surface of conduit 532. Such insulating material may include, for example, minerals, ceramics, etc. Conduit 532 may have critical flow orifices 515 disposed along its length within opening 514. Critical flow orifices 515 may be configured as described herein.

Electrical current may be applied to insulated conductor 562 to generate radiant heat in opening 514.

Conduit 532 may be configured to serve as a return for current. Insulated conductor 562 may be configured to heat portion 518 of the formation to a temperature sufficient to support oxidation of hydrocarbons.

Portion 518, reaction zone 524, and selected section 526 may have characteristics as described herein. Such a temperature may include temperatures as described herein.

Oxidizing fluid source 508 may provide oxidizing fluid into conduit 532. Oxidizing fluid may be provided into opening 514 through critical flow orifices 515 in conduit 532. Oxidizing fluid may oxidize at least a portion of the hydrocarbon containing formation in at reaction zone 524. Reaction zone 524 may have characteristics as described herein. Heat generated at reaction zone 524 may transfer heat to selected section 526, for example, by convection, radiation, and/or conduction. Oxidation product may be removed through a separate conduit placed in opening 514 or through an opening 543 in overburden casing 541. The separate conduit may be configured as described herein.

Packing material 542 and reinforcing material 544 may be configured as described herein.

FIG. 4 illustrates an embodiment of a natural distributed combustor heater with an added fuel conduit.

Fuel conduit 536 may be disposed into opening 514. It may be disposed substantially adjacent to conduit 533 in certain embodiments. Fuel conduit 536 may have critical flow orifices 535 along its length within opening 514.

Conduit 533 may have critical flow orifices 515 along its length within opening 514. Critical flow orifices 515 may be configured as described herein. Critical flow orifices 535 and critical flow orifices 515 may be placed on fuel conduit 536 and conduit 533, respectively, such that a fuel fluid provided through fuel conduit 536 and an oxidizing fluid provided through conduit 533 may not substantially heat fuel conduit 536 and/or conduit 533 upon reaction. For example, the fuel fluid and the oxidizing fluid may react upon contact with each other, thereby producing heat from the reaction. The heat from this reaction may heat fuel conduit 536 and/or conduit 533 to a temperature sufficient to substantially begin melting metallurgical materials in fuel conduit 536

and/or conduit 533 if the reaction takes place proximate to fuel conduit 536 and/or conduit 533. Therefore, a design for disposing critical flow orifices 535 on fuel conduit 536 and critical flow orifices 515 on conduit 533 may be provided such that the fuel fluid and the oxidizing fluid may not substantially react proximate to the conduits. For example, conduits 536 and 533 may be mechanically coupled such that orifices are oriented in opposite directions, and such that the orifices face the formation 516.

Reaction of the fuel fluid and the oxidizing fluid may produce heat. The fuel fluid and the oxidizing fluid may have characteristics herein. The fuel fluid may be, for example, natural gas, ethane, hydrogen or synthesis gas that is generated in the in situ process in another part of the formation. The produced heat may be configured to heat portion 518 to a temperature sufficient to support oxidation of hydrocarbons. Upon heating of portion 518 to a temperature sufficient to support oxidation, a flow of fuel fluid into opening 514 may be turned down or may be turned off. Alternatively, the supply of fuel may be continued throughout the heating of the formation, thereby utilizing the stored heat in the carbon to maintain the temperature in opening 514 above the autoignition temperature of the fuel.

The oxidizing fluid may oxidize at least a portion of the hydrocarbons at reaction zone 524. Generated heat will transfer heat to selected section 526, for example, by radiation, convection, and/or conduction. An oxidation product may be removed through a separate conduit placed in opening 514 or through an opening 543 in overburden casing 541. Packing material 542 and reinforcing material 544 may be configured as herein.

FIG. 5 illustrates an embodiment of a system configured to heat a hydrocarbon containing formation.

Electric heater 510 may be disposed within opening 514 in hydrocarbon containing formation 516. Opening 514 may be formed through overburden 540 into formation 516.

Opening 514 may be at least about 5 cm in diameter.

Opening 514 may, as an example, have a diameter of about 13 cm. Electric heater 510 may heat at least portion 518 of hydrocarbon containing formation 516 to a temperature sufficient to support oxidation (e. g., about 260 °C).

Portion 518 may have a width of about 1 m. An oxidizing fluid (e. g., liquid or gas) may be provided into the opening through conduit 512 or any other appropriate fluid transfer mechanism. Conduit 512 may have critical flow orifices 515 disposed along a length of the conduit.

Critical flow orifices 515 may be configured as described herein.

For example, conduit 512 may be a pipe or tube configured to provide the oxidizing fluid into opening 514 from oxidizing fluid source 508. For example, conduit 512 may be a stainless steel tube. The oxidizing fluid may include air or any other oxygen containing fluid (e. g., hydrogen peroxide, oxides of nitrogen, ozone). Mixtures of oxidizing fluids may be used. An oxidizing fluid mixture may include, for example, a fluid including fifty percent oxygen and fifty percent nitrogen. The oxidizing fluid may also, in some embodiments, include compounds that release oxygen when heated as described herein such as hydrogen peroxide. The oxidizing fluid may oxidize at least a portion of the hydrocarbons in the formation.

In some embodiments, a heat exchanger disposed external to the formation may be configured to heat the oxidizing fluid. The heated oxidizing fluid may be provided into the opening from (directly or indirectly)

the heat exchanger. For example, the heated oxidizing fluid may be provided from the heat exchanger into the opening through a conduit disposed in the opening and coupled to the heat exchanger. In some embodiments the conduit may be a stainless steel tube. The heated oxidizing fluid may be configured to heat, or at least contribute to the heating of, at least a portion of the formation to a temperature sufficient to support oxidation of hydrocarbons. After the heated portion reaches such a temperature, heating of the oxidizing fluid in the heat exchanger may be reduced or may be turned off.

FIG. 6 illustrates another embodiment of a system configured to heat a hydrocarbon containing formation.

Heat exchanger 520 may be disposed external to opening 514 in hydrocarbon containing formation 516.

Opening 514 may be formed through overburden 540 into formation 516. Heat exchanger 520 may provide heat from another surface process, or it may include a heater (e. g., an electric or combustion heater). Oxidizing fluid source 508 may provide an oxidizing fluid to heat exchanger 520. Heat exchanger 520 may heat an oxidizing fluid (e. g., above 200 °C or a temperature sufficient to support oxidation of hydrocarbons). The heated oxidizing fluid may be provided into opening 514 through conduit 521. Conduit 521 may have critical flow orifices 515 disposed along a length of the conduit.

Critical flow orifices 515 may be configured as described herein. The heated oxidizing fluid may heat, or at least contribute to the heating of, at least portion 518 of the formation to a temperature sufficient to support oxidation of hydrocarbons. The oxidizing fluid may oxidize at least a portion of the hydrocarbons in the formation.

In another embodiment, a fuel fluid may be oxidized in a heater located external to a hydrocarbon containing formation. The fuel fluid may be oxidized with an oxidizing fluid in the heater. As an example, the heater may be a flame-ignited heater. A fuel fluid may include any fluid configured to react with oxygen. An example of a fuel fluid may be methane, ethane, propane, or any other hydrocarbon or hydrogen and synthesis gas. The oxidized fuel fluid may be provided into the opening from the heater through a conduit and return to the surface through another conduit in the overburden. The conduits may be coupled within the overburden. In some embodiments, the conduits may be concentrically placed.

The oxidized fuel fluid may be configured to heat, or at least contribute to the heating of, at least a portion of the formation to a temperature sufficient to support oxidation of hydrocarbons. Upon reaching such a temperature, the oxidized fuel fluid may be replaced with an oxidizing fluid. The oxidizing fluid may oxidize at least a portion of the hydrocarbons at a reaction zone within the formation.